Various types of infra-red sensing clinical thermometers are known. U.S. Pat. No. 2,920,485 describes the use of a pyrometer and a chopper which is operative to intermittently expose a first temperature sensitive detector to radiation to be measured and a second temperature sensitive detector to a known reference temperature source.
U.S. Pat. No. 3,766,781 describes temperature sensing apparatus including a circular chopper, a reference thermistor and a thermal detector, as well as apparatus for converting the output voltage of the thermal detector into pulses.
U.S. Pat. No. 4,797,840 describes an infrared electronic thermometer and method for measuring temperature which employs a pyroelectric sensor and a shutter assembly and calculates the temperature of an object from the sensed change in temperature detected by the sensor upon opening of the shutter and the ambient temperature of the detector.
Additional patents which are relevant to the subject matter of the present invention include U.S. Pat. No. 5,017,018, describing a clinical thermometer which employs a lens and a concave mirror for directing infra-red radiation to the detector; U.S. Pat. No. 5,018,872 which describes a probe assembly for an infrared thermometer for operative engagement with an ear canal; U.S. Pat. No. 4,743,122, which describes infrared ray temperature measuring apparatus and including a distance keeping member; U.S. Pat. No. 4,790,324 which describes a method and apparatus for measuring internal body temperature utilizing infrared emissions; and U.S. Pat. No. 4,602,642, which describes a method and apparatus for measuring internal body temperature utilizing infrared emissions and employs a thermopile.
Most of the above-mentioned techniques use temperature change sensitive detectors which are very sensitive to the chopping frequency. This leads to bulky and complicated chopper means that has to be accurately controlled to maintain a constant chopping frequency in order to achieve constant responsivity from the detection unit. Other techniques bypass the problem by using other types of detectors or one-pulse detection, which reduces the system precision or complicates the technical solution. The present invention seeks to provide a simultaneous solution to the chopping frequency accuracy, volume of the chopping mechanism and complexity of the apparatus.
The teachings of the above-mentioned prior art documents are hereby incorporated by reference.